Full measure container

ABSTRACT

A container is provided that includes an upper chamber defined by upper sidewalls that together define a frusto-tetrahedral shape. Each of the upper sidewalls has an outwardly facing surface and at least one upper hollow pad protruding outwardly from the outwardly facing surface so that the at least one upper hollow pad increases the total enclosed and structural stiffness of the upper chamber. A lower chamber is also included that is defined by lower sidewalls that together form a frusto-tetrahedral shape. Each of the lower sidewalls has an outwardly facing surface and at least one lower hollow pad protruding outwardly from the outwardly facing surface wherein the at least one lower hollow pad increases the total enclosed volume and structural stiffness of the lower chamber. The upper chamber and lower chamber are interconnected by a central chamber defined by sidewalls that are recessed relative to the upper sidewalls and the lower sidewalls.

FIELD OF THE INVENTION

The present invention generally relates to bottles and containers and, more particularly to blow molded beverage bottles for delivery of beverages to consumers.

BACKGROUND OF THE INVENTION

Plastic bottles have been widely used for a variety of liquids as they are light in weight, have high shock resistance, are very cost effective to manufacture, and are able to be manufactured in extraordinarily high volumes of billions of units per year. Particularly, bottles molded by biaxially blow stretching a thermoplastic polyester such as a polyethylene terephthalate not only have good transparency and surface gloss, but are also equipped with the shock resistance, strength, and gas barrier characteristics required for the transportation and dispensing of liquids, such as juice, soft drinks, carbonated beverages, and the like.

Several bottles have been developed which include a self-supporting base molded into the bottle. One such bottle is disclosed in U.S. Pat. No. 3,598,270 which illustrates what is known as a “petaloid” design. The petaloid base design may be used for many different volume bottles including one, two, and three liter, twenty-four ounce, twenty ounce, half liter, twelve ounce and other sizes. The external configurations of petaloid base designs include many different shapes, however, a cylindrical container is widely used for the highly popular one, two, and three liter containers. The petaloid base design requires greater material thickness in the base portion of the bottle than in the side walls. Additionally, the diameter of the contact points is relatively small, limiting the stability of the bottle. Additional prior art petaloid designs may be found in U.S. Pat. Nos. 5,507,402; RE35,140; 5,482,170; 4,867,323; 4,465,199; and 4,140,241.

All of the forgoing blow molded synthetic resin containers have been suggested for use in many applications, and are produced in a variety of sizes and configurations, very often cylindrical in cross-section. During transport, a quantity of product containers are usually nested in an easily handled, larger rectangular shipping/handling container referred to in the art as a “shell.” Shells are often organized in stable loaded configurations on pallets, and may be used purely as a means to assist replenishment of retail store shelving or may additionally be used to stack the contained bottles in tiers, building retail floor displays at convenient locations within a retail store. When placed on typical rectangular retail store shelving, the cylindrical petaloid base design bottle is arranged in rows side to side and front to back. The bottles are tangent to one another and hence create gaps of unusable space on retail store shelving. The packing of cylindrical containers in rectangular shipping boxes or on the rectangular shelves of a retail display case often leaves large gaps between the product containers. These same gaps define available volume on rectangular retail store shelving which is not usable with cylindrical containers, limiting saleable container quantities per shelf.

The gaps between containers may be reduced, and almost eliminated, by using containers with rectangular cross-sections, since they may be placed closely together on retail store shelving side by side and front to back, hence no tangential gap remains to consume valuable space. However, rectangular containers will sometimes have a relatively larger footprint in order to define an equivalent internal volume as the cylindrical containers they replace, for a fully loaded retail store shelf.

Prior art two liter circularly cylindrical carbonated beverage bottles having a petaloid base are essentially uniform throughout the markets they serve. Differences between configurations generally are dimensional. Major consumer package goods companies have standardized dimensions and geometries for their respective petaloid base bottles. Generally, prior art petaloid bottles are 4.27 inches in diameter. Product brand labels aid in selection and other than bottle or fluid content color are the only differentiation. Prior art two liter carbonated beverage bottles may be arranged on typical flat retail shelves measuring twenty inches deep (d)×forty eight inches wide (w), as four bottles deep by eleven bottles wide. (forty four bottles per shelf). The marketplace also uses shelves which are slanted downward or have merchandising equipment installed which provides a way to use gravity to “front” the bottle providing access for the shopper. Gravity devices usually have a divider which aligns the prior art bottles for smooth sliding. The prevalent and popular gravity retail display stand in the U.S. market is constructed of wire, and co-extruded silicone and styrene plastic, with a wire divider that requires about one-quarter of an inch between bottle lanes thus allowing only four deep by ten wide bottles of the standard 4.27 inches diameter prior art two liter petaloid carbonated beverage bottle to be placed on the gravity device in a 48 inch span.

Additionally, prior art petaloid two liter carbonated beverage bottles have a positive pressure charge of carbonation entrained in the beverage solution. The removal of the finishing cap of the container allows the familiar whoosh of escaping carbonation. One result of this escape of the effervescent gas is the de-rigidization of the two liter carbonated beverage petaloid bottle with a concomitant significant reduction in column strength. These prior art petaloid bottles become limber and more awkward to handle since the majority of their inherent column strength and rigidity was created by their pressurization. In addition, the small ring at the finishing neck of a prior art two liter carbonated beverage bottle is frequently difficult for smaller hands to maneuver during pouring of the beverage. Prior art two liter petaloid bottles require great strength to control, especially using the neck ring when the bottle is uncapped and no longer rigid.

As a consequence, there has been a long felt need for a retail container that satisfies the foregoing concerns in the art by allowing denser packing in a retail display stand while also providing improved column strength when in an unpressurized condition.

SUMMARY OF THE INVENTION

The present invention provides a full measure container that includes an upper chamber defined by upper sidewalls that together define a frusto-tetrahedral shape. Each of the upper sidewalls has an outwardly arcing surface and at least one upper hollow pad protruding outwardly from the outwardly arcing surface so that the at least one upper hollow pad increases the total enclosed volume of the upper chamber and also provides additional stiffening structure to the upper chamber. A lower chamber is also includes that is defined by lower sidewalls that together form a frusto-tetrahedral shape. Each of the lower sidewalls has an outwardly arcing surface and at least one lower hollow pad protruding outwardly from the outwardly arcing surface wherein the at least one lower hollow pad increases the total enclosed volume of the lower chamber and also provides additional stiffening structure to the lower chamber. The upper chamber and lower chamber are interconnected by a central chamber defined by sidewalls that are recessed relative to the upper sidewalls and the lower sidewalls.

In another embodiment, a blow molded synthetic resin container is provided that includes an upper chamber defined by outwardly arced and upwardly swept upper sidewalls that together define a frusto-tetrahedral volume. Each of the upper sidewalls has an outwardly arcing surface and two upper hollow pads protruding outwardly from the outwardly arcing surface. A lower chamber is provided that is defined by outwardly arced and upwardly swept lower sidewalls that together define a frusto-tetrahedral volume. Each of the lower sidewalls comprise an outwardly arcing surface and two lower hollow pads protruding outwardly from the outwardly arcing surface. A central waist chamber is defined by sidewalls that are recessed relative to the upper sidewalls and the lower sidewalls.

In a further embodiment, a polymeric container is provided which exhibits superior top load strength. The container includes an upper chamber defined by four outwardly arced and upwardly swept upper sidewalls that together define a frusto-tetrahedral volume. Each of the upper sidewalls comprise an outwardly arcing surface having two hollow tapered and outwardly arced pads that protrude from the outwardly arcing surface thereby defining two hollow alcoves in each inwardly facing surface of each upper sidewall. A lower chamber is provided that is defined by outwardly arced and upwardly swept lower sidewalls that together define a frusto-tetrahedral volume. Each of the lower sidewalls comprise an outwardly arcing surface having two hollow tapered and outwardly arced pads that protrude from the outwardly arcing surface thereby defining two hollow alcoves in each inwardly facing surface of each lower sidewall. A central chamber is defined by sidewalls that are recessed relative to the upper sidewalls and the lower sidewalls.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiments of the invention, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:

FIG. 1 is a perspective view of a plurality of prior art carbonated beverage bottles positioned upon a standard retail display gravity shelf;

FIG. 2 is a perspective view of a full measure bottle with volume compensation/stiffening pads formed according to the invention;

FIG. 3 is a transverse cross-sectional view of a upper chamber of the full measure bottle shown in FIG. 2;

FIG. 4 is a transverse cross-sectional view of a lower chamber of the full measure bottle shown in FIG. 2;

FIG. 5 is a longitudinal cross-sectional view of a upper chamber of the full measure bottle shown in FIG. 2;

FIG. 6 is a top view of the full measure bottle with volume compensation/stiffening pads;

FIG. 7 is a bottom view of the full measure bottle with volume compensation/stiffening pads; and

FIG. 8 is a perspective view of a plurality of full measure bottles, each with multiple volume compensation/stiffening pads formed in accordance with the invention positioned upon a standard retail display gravity shelf.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This description of preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. The drawing figures are not necessarily to scale and certain features of the invention may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. In the description, relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. In the claims, means-plus-function clauses, if used, are intended to cover the structures described, suggested, or rendered obvious by the written description or drawings for performing the recited function, including not only structural equivalents but also equivalent structures.

Referring to FIGS. 2-5, a full measure bottle 2 formed in accordance with one embodiment of the present invention comprises a thin-walled polymer container having an upper chamber 3, a waist 5, a lower chamber 7, and a bottom wall 9 (FIG. 7). Upper chamber 3 is defined by polygonally shaped sidewalls 10 each having an outwardly arcing surface 11 and inwardly facing surface 13. Lower chamber 7 is also defined by polygonally shaped sidewalls 15 each having an outwardly arcing surface 17 and inwardly facing surface 19. Waist 5 is disposed between upper chamber 3 and lower chamber 7. Waist 5 often comprises a generally polygonal cross-sectional shape including sidewalls 16 that are each recessed inwardly relative to sidewalls 10 of upper chamber 3 and sidewalls 15 of lower chamber 7 so as to form a narrowed chambered portion of full measure bottle 2 that is suitable for gripping by hand. Bottom wall 9 closes the container and is normally no less than the equivalent in length and width of lower chamber 7 so as to define a “foot-print” 18 for full measure bottle 2.

In some preferred embodiments bottom 9 is somewhat larger in length and width than sidewalls 15 of lower chamber 7 so that side walls 15 and bottom 9 together define a frusto-tetrahedrally shaped lower chamber 7. This same frusto-tetrahedral shape is also often incorporated in upper chamber 3 as well. In these embodiments, footprint 18 is often somewhat less than four inches by four inches, e.g., about 3.692 inches by about 3.692 inches front to back and side to side. This is typically the case since full measure bottle 2 is preferably not truly rectangular in transverse profile. Instead, upper chamber 3 and lower chamber 7 form a pair of truncated and outwardly bowed or arced pyramidal volumes, i.e., frusto-tetrahedrally shaped having outwardly bowed sidewalls 10,15, which taper upwardly such that sidewalls 10,15 define upwardly and somewhat outwardly swept arcs (FIG. 2). In other words, upper chamber 3 defines and encloses a substantially frusto-tetrahedrally shaped volume above waist 5 and another, larger substantially frusto-tetrahedrally shaped volume is formed by lower chamber 7 that is capped by bottom wall 9.

Each of upper chamber 3 and lower chamber 7 is advantageously constructed so as to approach the volume equivalent of a circularly-cylindrical beverage container, e.g., a circularly-cylindrical two liter carbonated soft-drink bottle, having a diameter that is approximately the same length as the respective largest internal diagonal width of each of upper chamber 3 and lower chamber 7, hereinafter referred to as a “circularly-cylindrical equivalent volume” or “CCEV.” The CCEV is advantageously obtained in full measure bottle 2 by the formation of one or more tapered, arced protrusions or pads 22 that protrude outwardly from outwardly arcing surfaces 11,17 thereby defining hollows or alcoves 24 in inwardly facing surfaces 13,19 of sidewalls 10 and sidewalls 15.

More particularly, each sidewall 10 of upper chamber 3 has formed in outwardly arcing surfaces 11 outwardly protruding, tapered pads 22 that often have a prismatic shape, and are formed by outwardly projecting walls 23 and face wall 25. Outwardly projecting walls 23 are often tapered so as to be wider toward the top of upper chamber 3 and narrower toward waist 5. Walls 23 may also be arranged at an obtuse angle of about 91° to about 93° relative to a transverse dimension of sidewall 10 so as to provide draft for removal of full measure bottle 2 from its mold. Walls 23 and face 25 define the boundaries of the volume created by alcoves 24 within inwardly facing surfaces 13. Each of sidewalls 10 preferably include two side-by-side pads 22, making a total of eight pads 22 protruding from outwardly arcing surfaces 11 of side walls 10 on upper chamber 3.

Additionally, each sidewall 15 of upper chamber 7 has formed in outwardly arcing surfaces 17 outwardly protruding, tapered pads 22 that also often have a prismatic shape, and are formed by outwardly projecting walls 27 and face wall 29. Outwardly projecting walls 27 are often tapered so as to be wider toward waist 5 and narrower toward bottom 9. Walls 27 may also be arranged at an obtuse angle of about 91° to about 93° relative to a transverse dimension of sidewall 15 so as to provide draft for removal of full measure bottle 2 from its mold. Walls 27 and face 29 define the boundaries of the volume created by alcoves 24 within inwardly facing surfaces 19. Each of sidewalls 15 preferably include two side-by-side pads 22, making a total of eight pads 22 protruding from outwardly arcing surfaces 17 of side walls 15 on lower chamber 7. Each of pads 22 protrudes outwardly sufficiently to define additional available volume within each of alcoves 24 so that when added to the CCEV of upper chamber 3 and lower chamber 7 they bring the total volume available in full measure bottle 2 to substantially equivalent to a circularly-cylindrical volume of a typical prior art two liter carbonated soft-drink bottle.

Thus, full measure bottle 2 has rigidity and improved column strength (when compared to prior art circularly-cylindrical petaloid carbonated beverage bottles) that is provided by frusto-tetrahedrally shaped upper and lower vessels 3,7 and the arced sidewalls 10,15, and profile of pads 22 which also provides for a greater moment of inertia. Advantageously, full measure bottle 2 does not solely rely on the positive pressure of carbonation to provide rigidity of the container. Moreover, the reduced size of waist 5 which is adapted for hand and label placement, is also natural for the hand to grasp. The hand grasp location near the center of gravity of waist 5 also improves control during pouring of beverage from the full measure bottle 2.

The stability of full measure bottle 2 is improved when compared to prior art two liter carbonated beverage bottles since some preferred embodiments possess a wider footprint 18 when compared to prior art circularly-cylindrical two liter petaloid base carbonated beverage bottles. In particular, the five petals of the base of a prior art two liter carbonated beverage bottle are inscribed within a circumference of 2.72″ or about 63% of the bottle diameter. Advantageously, the often preferred 3.692 inch per side length of full measure bottle 2 at its base may be a maximum dimension of full measure bottle 2. Prior art petaloid instability is a common experience during shopping, replenishment, and use in the home and thus full measure bottle 2 provides a useful improvement.

An open neck 30 projects upwardly from a top wall 32 of upper chamber 3, and is sized and structured to receive a closure cap 33 for sealing full measure bottle 2. Open neck 30 allows liquids or flowable solid materials to enter into full measure bottle 2 and to be removed when desired, typically by the consumer of the contents of the container. Advantageously, full measure bottle 2 is well-adapted for storing, transporting, displaying and dispensing liquids or flowable solid materials. Full measure bottle 2 is especially useful for storing, transporting and displaying selected quantities of liquid products, preferably carbonated beverages, although it would be quite suitable for storing other beverages, such as fruit juices, water, dairy products, and the like, as well as, more viscous food products, such as condiments and non-solid food products. Significantly, due to its stacked, frusto-tetrahedrally shape each of which both arcs and tapers upwardly such that sidewalls 10,15 define upwardly swept arcs, full measure bottle 2 comprises a superior column strength when compared to traditional, circularly-cylindrical carbonated beverage bottles, particularly when in an opened and unpressurized state.

When full measure bottle 2 is in use as a consumer-directed package for carbonated beverages, footprint 18 advantageously allows a retail pack out of five bottles deep by thirteen bottles wide or sixty-five bottles on a flat retail shelf (FIG. 8) in contrast to a retail pack out of forty-four prior art bottles. Hence, the smaller footprint of full measure bottle 2 is very desirable and useful. Full measure bottle 2 also offers pack out improvement of one hundred and forty-seven point seven percent (65 bottles/44 bottles equals 147.7%) on flat shelves in the eastern U.S. market. The more efficient full measure bottle 2 also affords improved trailer loading and fewer trips thus reducing fuel consumption for store delivery. Full measure bottle 2 also reduces the time spent in the restocking retail store shelves by reducing the number of visits by direct store delivery personnel, reducing the number of stock outs, all while improving capacity of the existing retail store shelves.

As with other thin walled polymer containers, full measure bottle 2 may be blow molded from a preform or “parison,” i.e., a hollow plastic melt tube that is extruded from a die head of a conventional blow molding machine so as to be suitable for expansion within a mold (not shown). Polymeric materials useful in this invention include any material that is suitable for use in the food and beverage packaging industry, and suitable for injection molding and injection stretch blow molding, for example, polyethylene terephthalate, polyolefins, polypropylene, polyethylene naphthalate, polyvinyl chloride, and others, with polyethylene terephthalate. In the preferred manufacturing arrangement, the parison may be preformed by injection molding or the like, and then subjected to blow molding procedures, typically incorporating stretch blow molding techniques followed by heat setting. Recesses within the mold that correspond in position, depth, taper, and arcs of pads 22 fill with the expanding polymer during the blow molding process. The depth, size, and shape of these recesses may be adjusted so as to allow for the formation of a full measure bottle 2 having a wide variety of volumes, but always possessing the same “foot-print” 18. Advantageously, retaining the same “foot-print” 18 allows for storing, transporting and displaying selected quantities of liquid products, preferably carbonated beverages, in traditional racks or shelves.

ADVANTAGES OF THE INVENTION

Numerous advantages are obtained by employing the present invention.

More specifically, a novel, blow molded synthetic resin container is provided which may be closely nested with like containers to reduce or eliminate space therebetween so as to avoid all of the aforementioned problems associated with prior art containers.

Furthermore, a novel, blow molded synthetic resin container is provided which may be closely nested with like containers to reduce or eliminate space therebetween during retail display while increasing the total number of containers displayed, e.g., the surface area of a cylindrical bottle is more mathematically efficient but the footprint area is not. A cylindrical bottle often occupies a footprint of 18.2329 square inches. The cross-sectional area of a standard two liter petaloid bottle is 14.320085 square inches, while a two liter full measure bottle formed in accordance with the present invention has a footprint that is about 13.63086 square inches.

Additionally, a novel, blow molded synthetic resin container is provided which may be closely nested with like containers to reduce or eliminate space therebetween but having superior column strength when compared to prior art circularly-cylindrical carbonated beverage bottles.

Also, a novel, blow molded synthetic resin container is provided which may be closely nested with like containers to reduce or eliminate space therebetween and that is constructed so as to approach the volume equivalent of a circularly-cylindrical beverage container, e.g., a circularly-cylindrical two liter carbonated soft-drink bottle.

Additionally, a novel, blow molded synthetic resin container is provided which may be closely nested with like containers to reduce or eliminate space therebetween and that has greatly improved column strength due to a stacked, frusto-tetrahedral construction.

In addition, a novel, blow molded synthetic resin container is provided which may be closely nested with like containers to reduce or eliminate space therebetween and that is “heftier” and having a distinctive shape and efficient relatively small footprint that allows for a maximized volume.

Another advantage is provided by the novel, blow molded synthetic resin container of the present invention since a future gravity display stand for standard retail shelves is being considered that is formed as an injection molded plastic stand with a divider which uses only 3/16 of an inch between bottle lanes allowing a pack out of five deep by twelve wide (sixty bottles) in a forty-eight inch span.

It is to be understood that the present invention is by no means limited only to the particular constructions herein disclosed and shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims. 

1. A container comprising: an upper chamber defined by upper sidewalls that together define a frusto-tetrahedral shape, each of said upper sidewalls comprises an outwardly arcing surface and at least one upper hollow pad protruding outwardly from said outwardly arcing; a lower chamber defined by lower sidewalls that together define a frusto-tetrahedral shape, each of said lower sidewalls comprises an outwardly arcing surface and at least one lower hollow pad protruding outwardly from said outwardly arcing surface; and a central chamber defined by sidewalls that are recessed relative to said upper sidewalls and said lower sidewalls.
 2. A container according to claim 1 wherein said upper sidewalls are polygonally shaped each presenting an outwardly arcing surface.
 3. A container according to claim 1 wherein said lower sidewalls are polygonally shaped each presenting an outwardly arcing surface.
 4. A container according to claim 1 wherein said central chamber defines a narrow waist that is disposed between said upper chamber and said lower chamber, and comprises a generally polygonal cross-sectional shape.
 5. A container according to claim 1 comprising a bottom wall located across a bottom of said lower chamber that closes said container and which is normally no less than the equivalent in length and width of said lower chamber so as to define a foot-print of said container.
 6. A container according to claim 5 wherein said bottom wall is somewhat larger in length and width than said lower sidewall so that said side walls and said bottom together define a frusto-tetrahedrally shaped lower chamber.
 7. A container according to claim 5 wherein said footprint is less than four inches by four inches.
 8. A container according to claim 5 wherein said footprint is about 3.692 inches by about 3.692 inches.
 9. A container according to claim 1 wherein said upper sidewalls and said lower sidewalls are outwardly arced and upwardly swept so as to define and enclose a frusto-tetrahedrally shaped volume above said waist and a frusto-tetrahedrally shaped volume, larger frusto-tetrahedrally shaped volume below said waist.
 10. A container according to claim 1 wherein said upper chamber, said waist, and said lower chamber define a volume that is substantially equivalent to a circularly-cylindrical container.
 11. A container according to claim 1 wherein said container includes at least one tapered and outwardly arced pad that protrudes from an outwardly arcing surface of at least one of an upper sidewall and a lower sidewall thereby defining at least one hollow alcove in an inwardly facing surface.
 12. A container according to claim 1 wherein said upper chamber includes an outwardly arcing surface and two outwardly protruding and tapered pads.
 13. A container according to claim 12 wherein said two outwardly protruding and tapered pads are formed by at least two outwardly projecting walls and a face wall wherein said outwardly projecting walls are tapered so as to be wider toward a top of said upper chamber and narrower toward said waist.
 14. A container according to claim 13 wherein said upper sidewalls preferably include two side-by-side pads, making a total of eight pads protruding from said outwardly arcing surfaces of said side walls on said upper chamber.
 15. A blow molded synthetic resin container having: an upper chamber defined by outwardly arced and upwardly swept upper sidewalls that together define a frusto-tetrahedral volume, each of said upper sidewalls comprises an outwardly arcing surface and two upper hollow pads protruding outwardly from said outwardly arcing surface; a lower chamber defined by outwardly arced and upwardly swept lower sidewalls that together define a frusto-tetrahedral volume, each of said lower sidewalls comprise an outwardly arcing surface and two lower hollow pads protruding outwardly from said outwardly arcing surface; and a central waist chamber defined by sidewalls that are recessed relative to said upper sidewalls and said lower sidewalls.
 16. A polymeric container which exhibits superior top load strength comprising: an upper chamber defined by four outwardly arced and upwardly swept upper sidewalls that together define a frusto-tetrahedral volume, each of said upper sidewalls comprise an outwardly arcing surface having two hollow tapered and outwardly arced pads that protrude from said outwardly arcing surface thereby defining two hollow alcoves in each inwardly facing surface of each upper sidewall so as to stiffen each of said upper sidewalls; a lower chamber defined by outwardly arced and upwardly swept lower sidewalls that together define a frusto-tetrahedral volume, each of said lower sidewalls comprises an outwardly arcing surface having two hollow tapered and outwardly arced pads that protrude from said outwardly arcing surface thereby defining two hollow alcoves in each inwardly facing surface of each lower sidewall so as to stiffen each of said lower sidewalls; and a central chamber defined by sidewalls that are recessed relative to said upper sidewalls and said lower sidewalls. 